This invention relates to AC drives and, in particular, the invention provides a method of stator flux oriented control for variable speed AC drives.
Existing variable speed AC drives are highly complicated and prone to nuisance trips. The complicated nature of existing drives translates into large amounts of engineering time in order to apply the drive to systems with unusual requirements. For the user, a complicated drive means complicated set up procedures, component failures, degradation, and drift.
Therefore, it is the primary objective of this invention to provide a control system for an AC drive which operates under all reasonably expected conditions of input, line, load, or customer adjustment.
A further objective of this invention is to provide a control system for an AC drive which operates independent of motor parameters.
An additional objective of this invention is to provide a control system for an AC drive which will operate without tripping under any conceivable load condition.
A still further objective of this invention is to provide a control system for an AC drive which is simple and low cost as compared to a conventional field oriented control which requires accurate knowledge of motor parameters and/or tachometer feedback.
A still further objective of this invention is to provide a control system for an AC drive which operates without a tachometer.
The method of stator flux oriented control to which this invention relates directs the switching signals from a waveform generator to an inverter supplying pulse width modulated current to a three-phase variable frequency induction motor. The present invention provides a simple implementation by using five proportional-integral regulators to control the AC drive, instead of the complex calculations required in prior art devices.
A three-phase, induction motor may be mathematically represented as a two-phase, induction motor having two axes of magnetic symmetry, direct axis and the quadrature axis. In order to mathematically translate the three-phase motor into a two-phase motor, the value of two of the three phase motor currents must be determined. A sensor is used to provide a reference frame converter with the values of the two phase currents. The reference frame converter mathematically calculates a quadrature axis stator current signal and a direct axis stator current signal from the two sensed phase currents and from a reference angle provided to the reference frame converter.
A flux estimator produces an estimated quadrature axis stator flux signal and an estimated direct axis stator flux signal. A flux reference generator produces a direct axis stator flux reference signal. The direct axis stator flux reference signal and the estimated direct axis stator flux signal are input into a flux amplitude regulator. The output of the flux amplitude regulator is a direct axis stator current reference signal.
The direct axis stator current signal and the direct axis stator current reference signal are inputted into a direct axis stator current regulator. The output of the direct axis stator current regulator is a direct axis stator voltage signal.
The estimated quadrature axis stator flux signal and a quadrature axis stator flux reference signal are input into a flux angle regulator. The output of the flux angular regulator is an estimate of the rotor speed of the induction motor.
The estimated rotor speed and a speed reference are inputted into a speed regulator. The output of the speed regulator is a quadrature axis stator current reference signal. A quadrature axis stator current regulator receives the quadrature axis stator current reference signal from the speed regulator and the quadrature axis stator current signal from the reference frame converter. The output of the quadrature axis stator current regulator is a quadrature axis stator voltage signal.
An estimated slip frequency is generated and combined with the estimated rotor speed signal to produce a stator frequency signal.
The direct axis stator voltage signal and the quadrature axis stator voltage signal are translated from rectangular coordinates to polar coordinates. This results in a voltage magnitude signal and a voltage angle signal. By adding the voltage angle signal to the integral of the stator frequency, a waveform reference angle signal is produced. The waveform reference angle signal and the voltage magnitude signal are inputted into the waveform generator. Responsive to these two signals, the waveform generator generates switching commands for the inverter. The waveform reference angle is also inputted into the reference frame converter as a necessary variable in calculating the two-phase motor representation of the three-phase motor.